diff --git a/doctrine-mapping.xsd b/doctrine-mapping.xsd
index b8e4de1a7..c0dbfafcc 100644
--- a/doctrine-mapping.xsd
+++ b/doctrine-mapping.xsd
@@ -257,7 +257,7 @@
     <xs:attribute name="target-entity" type="xs:string" use="required" />
     <xs:attribute name="field" type="xs:NMTOKEN" use="required" />
     <xs:attribute name="mapped-by" type="xs:NMTOKEN" />
-    <xs:attribute name="index-by" type="xs:MNTOKEN" />
+    <xs:attribute name="index-by" type="xs:NMTOKEN" />
     <xs:attribute name="inversed-by" type="xs:NMTOKEN" />
     <xs:attribute name="fetch" type="orm:fetch-type" default="LAZY" />
   </xs:complexType>
@@ -270,7 +270,7 @@
     <xs:attribute name="target-entity" type="xs:string" use="required" />
     <xs:attribute name="mapped-by" type="xs:NMTOKEN" use="required" />
     <xs:attribute name="field" type="xs:NMTOKEN" use="required" />
-    <xs:attribute name="index-by" type="xs:MNTOKEN" />
+    <xs:attribute name="index-by" type="xs:NMTOKEN" />
     <xs:attribute name="orphan-removal" type="xs:boolean" default="false" />
     <xs:attribute name="fetch" type="orm:fetch-type" default="LAZY" />
   </xs:complexType>